Sulfur-containing compounds are present to a greater or lesser extent in petroleum and in the refined products from petroleum. Organic sulfur compounds are undesirable in petroleum products for several reasons. They can poison catalysts used in petroleum refining. In addition, the greater the amount of organic sulfur compounds in hydrocarbon fuels, such as gasoline and heating oil, the greater the potential for pollution when these fuels are burned. Several methods have been developed over the years to deal with the problems posed by organic sulfur compounds in petroleum. These range from simple treatments aimed at rendering less offensive the malodorous sulfur compounds, to more complex processes to reduce or remove the sulfur compounds. The various methods used include absorption, extraction, oxidation hydro-desulfurization etc.
Desulfurization of heavy oils or petroleum crude oils using metallic sodium has been disclosed in several prior art processes.
U.S. Pat. No. 3,565,792 discloses an integrated process for the desulfurization of crude oil or similar virgin hydrocarbon fractions wherein a dispersion of metallic sodium is employed to react with the sulfur contaminants present within such crude oil to form a sodium sulfide precipitate eliminated from the treated crude through centrifugation. The integrated system is provided by reacting the sodium sulfide precipitate with hydrochloric acid to produce sodium chloride which after reclamation can be employed through electrolysis to provide the necessary sodium for desulfurization of the crude oil.
U.S. Pat. No. 3,755,149 discloses a process for desulfurizing vacuum bottoms of a short residue which consists of contacting said residue with metallic sodium at a temperature of from about 250° C. to about 400° C. and at a hydrogen pressure of from about 1 to about 100 atmospheres, extracting the sludge-containing product thus produced with an aliphatic hydrocarbon, separating the sludge from the hydrocarbon extract and removing said hydrocarbon to yield a desulfurized resid.
U.S. Pat. No. 6,210,564 discloses a process for desulfurization of petroleum feeds utilizing sodium metal. Sulfur-containing petroleum feeds are desulfurized by contacting the feeds with staged addition of sodium metal at temperatures of at least about 250° C. in the presence of excess hydrogen to sodium metal. The formation of Na2S is substantially suppressed and the formation of NaSH is promoted in the desulfurization process.
US2005145545 discloses desulfurization of petroleum streams using metallic sodium. The method of removing sulfur from a hydrocarbon feed involves the steps of dissolving metallic sodium in a solvent and combining the sodium/solvent solution with a liquid hydrocarbon feed containing an organosulfur species. The pressure of combination is above the vapor pressure of the solvent. The combined hydrocarbon feed and solvent solution are placed in a low pressure environment to vaporize the solvent. The resulting stream is combined with hydrogen gas and this stream is heated and pressurized to form a liquid hydrocarbon product containing sodium sulfide. This product is then cooled and the sodium sulfide is extracted.
A co-pending Indian Patent application No. 845/MUM/2011 discloses an improved process of desulfurization of petroleum residue by means of addition of a suitable organic solvent to the petroleum feed oil prior to the desulfurization reaction. The process results in the formation of sodium sulfide (Na2S) as a major by-product.
In general, the Na2S formed is filtered and separated from the desulfurized feed oil and is then either dissolved in water or subjected to Na recovery by electrolysis process. This by-product obtained is essentially in the form of a sticky sludge and may contain impurities such as un-reacted Na, residual feed oil and other organic sodium salts.
One of the major challenges is the separation and filtration of the by-product from that of the desulfurized oil. A substantial quantity of the feed oil remains associated with this Na2S by-product. The addition of water directly for the by-product recovery, doesn't result in complete dissolution of Na2S due to a water impenetrable coating of the residual feed oil on the by-product. This feed oil remaining associated with the Na2S, results in the formation of an emulsion on addition of water, probably due to the high viscosity and sticky nature of petroleum feed oil and similar specific gravities of water and oil. This leads to the formation of a non-separable mass which results in loss of valuable feed oil remaining associated to the by-product, as well as contamination of Na2S solution by the feed oil. Further, the recovery of oil from the by-product sludge can be carried by means of an organic solvent capable of dissolving the oil. However, a multistep process is required with addition of the fresh organic solvent at each step. This results in use of large quantities of organic solvent and thus adding to the cost of solvent recovery by distillation in addition to the solvent loss associated with the separation and distillation processes.
Still, further, the solvent is unable to completely recover the oil even by means increasing the extraction steps. This is because as the solvent dissolves the oil, fresh Na2S surfaces are exposed which are not soluble in the organic solvents. Thus, the oil that has remained entrapped by the solid Na2S surface doesn't get exposed to the organic solvent and hence the dissolution efficiency drops down drastically.
Accordingly, it is desirable to develop a simple process for separation and purification of sodium sulfide obtained as a major by-product from the process of desulfurization of petroleum oils.
Further, it is known that the use of the Na2S by-product in general is towards regeneration of Na by means of suitable electrolytic processes. The present disclosure is also aimed at value addition to this Na2S by-product by means of oxidation to Na2SO3 which is a higher value product.
Some of the representative patent documents which disclose methods for converting sodium sulfide into sodium sulfite are described herein below.
U.S. Pat. No. 3,165,378 discloses a method of converting sodium sulfide produced by preliminary treatment of spent liquor which is recovered from the chemical solution used in production of paper pulp from wood and the like and which contains sulfur compounds. The process comprises passing an aqueous solution containing sodium sulfide through a reaction zone in an atmosphere of saturated steam and air under substantially constant pressure within the range of 60 to 125 pounds per square inch.
U.S. Pat. No. 3,657,064 discloses direct oxidative conversion of sodium sulfide to sodium sulfite by absorbing the heat of reaction in a fluidized bed system using adiabatic cooling.
U.S. Pat. No. 3,698,860 discloses a process for converting sodium sulfide in the smelt resulted from the burning of black liquor from digesters of semichemical pulp and sulfite pulp to sodium sulfite by oxidation and recovering the latter as pulp digesting chemical, which comprises mixing the smelt particles with water, adding thereto a minor amount of sodium hydroxide forming the mixture into particles and introducing the particles into a converter packed with dry powder of sodium sulfite, sodium carbonate, etc., containing no sodium sulfide, the process being performed throughout as a wet process.
The processes disclosed in the prior art patent documents are complex and time consuming. Accordingly, it is desirable to develop a simple method for converting sodium sulfide into sodium sulfite which avoids utilization of catalyst.